The present invention relates to a vibration damping structure that is used in a steering apparatus of a vehicle to damp vibration of a steering wheel having an airbag device.
Vibration of a steering wheel in a vertical direction and a left-right direction, which may occur when the vehicle travels at a high speed or the engine of the vehicle idles, decreases driving comfort. Techniques for damping such vibration of the steering wheel thus nave been developed and proposed. One such technique employs a dynamic damper. The dynamic damper is configured by a weight acting as a damper mass and an elastic member for supporting the weight with respect to a component such as a metal core of a steering wheel. In this technique, the dynamic damper, which is attached to the steering wheel, receives vibration from the steering wheel at a frequency close or equal to the natural frequency of the dynamic damper. This causes the dynamic damper to resonate, thus absorbing vibration energy from the steering wheel. As a result, the vibration of the steering wheel is damped.
An airbag device is mounted in the pad of the steering wheel to protect the driver in a car crash. The airbag device includes an airbag and an inflator for supplying gas to the airbag. When a car crash happens, the airbag device sends gas from the inflator to the airbag to inflate the airbag rearward, thus protecting the driver from impact.
However, since a large space is necessary to install the airbag device in the interior of the steering wheel, it is difficult to mount the aforementioned dynamic damper in recent steering wheels.
To solve this problem, one proposed vibration damping structure for a steering wheel includes an inflator of an airbag device functioning as a damper mass of a dynamic damper (see, for example, Japanese Laid-Open Patent Publication No. 2006-96127). Specifically, the inflator of the steering wheel includes a cylindrical peripheral wall portion and gas ports formed in the peripheral wall portion. By discharging gas outward from the gas ports in a radial direction of the peripheral wall portion, the inflator inflates the airbag rearward. A support member having an elastic support portion for elastically supporting the inflator is arranged in front of the inflator. The inflator thus functions as the damper mass of the dynamic damper and the elastic support portion functions as a spring in the dynamic damper.
A communication hole is formed in the support member at a position facing the inflator. Annular sealing portions are arranged in the inflator and the support member at positions surrounding the communication hole. The inflator thus contacts the support member through the sealing portions, thus restricting gas leakage through the communication hole.
In the steering wheel described in Japanese Laid-open Patent Publication No. 2006-96127, the inflator is prevented from contacting the support member through the sealing portions when the airbag is not inflated. If the dynamic damper receives (the inflator and the elastic support portion receive) vibration from the steering wheel at a frequency close or equal to the natural frequency of the dynamic damper, the elastic support portion elastically deforms and vibrates together with the inflator. The dynamic damper thus absorbs vibration energy from the steering wheel and damps vibration of the steering wheel.
When the vehicle receives impact, gas is discharged outward from the gas ports, which is formed in the peripheral wall portion of the inflator, in the radial direction of the peripheral wall portion. The gas is thus fed into the airbag to inflate the airbag. This increases the pressure in the airbag, thus applying force acting forward of the inflator. The force causes the inflator to elastically deform the elastic support portion and move forward, thus contacting the support member through the sealing portions. This seals the space between the inflator and the support member, thus preventing gas leakage through the communication hole.
However, in the steering wheel described in Japanese Laid-Open Patent Publication No. 2006-96127, prevention of gas leakage from the communication hole is brought about only after the airbag is inflated to a certain extent. Specifically, the force acting forward is applied to the inflator only after the pressure in the airbag rises to a certain level. As a result, the space between the inflator and the support member is maintained unsealed in the period from when the gas starts being discharged from the inflator to when the pressure in the airbag rises to the certain level. This may cause leakage of the gas through the communication hole.